I have a quadrotor aircraft (4 motors/propellers around a center housing) with some electronics in an electronics housing. Looking to cool the electronics with propeller downwash.

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I have a vent on the side of the electronics housing and one on the bottom. I assume the pressure on bottom P0 is atmospheric. Trying to determine which way the air will flow.

Would pressure P1 be higher or lower than atmospheric?

The propellers do work on the fluid and increase the dynamic pressure but there is also air at high velocities flowing past the side vents. Having trouble consolidating propeller dynamics with Bernoulli's principle.

  • $\begingroup$ Why not add a little scoop to direct air into the enclosure? $\endgroup$
    – hazzey
    Dec 11, 2015 at 23:44
  • $\begingroup$ That is is something I am considering but I am unsure of how much air is flowing downward there. The flow field around the tips of the propellers is a bit messy. $\endgroup$
    – pdfj
    Dec 12, 2015 at 0:41
  • $\begingroup$ The question can be clarified a little bit, to state that it is a theoretical question for a hypothetical design. As it stands now, it can be suggested that it may not be necessary to use propeller stream. (heated air around the electronics will wash away, anyway) $\endgroup$ Dec 13, 2015 at 7:40

1 Answer 1


It depends on the velocity across P1 and the pressure differential across your blades. It would be hard to determine without testing or using computation fluid dynamics. That is probably overkill and its much easier to just change the geometry to do what you need. Hazzey mentioned a scoop in the comments; that would work well but it would also send rain and dust through. You can also do it the opposite way by putting down facing outlet to generate negative pressure. This would allow you to have a large area on the bottom vent and be able to filter the intake air.

  • $\begingroup$ Hmm that is interesting with directing scoop downward. So your saying air naturally flows in and builds up pressure and this built up pressure releases from downscoop? $\endgroup$
    – pdfj
    Dec 12, 2015 at 0:49
  • $\begingroup$ @pdfj no the airflow of the prop will pull air with it after the scoop this creates a pressure drop in the scoop which gets rebalanced by air from inside the housing $\endgroup$ Dec 12, 2015 at 15:09
  • $\begingroup$ @ratchetfreak so what is the mechanism for the airflow of the prop pulling air with it. $\endgroup$
    – pdfj
    Dec 12, 2015 at 22:55
  • $\begingroup$ @pdfj the venturi effect $\endgroup$ Dec 12, 2015 at 22:58
  • $\begingroup$ doesn't the prop increase the total pressure though by imparting kinetic energy into the flow? so the flow past the scoop could be higher total pressure than outside the bottom vent? $\endgroup$
    – pdfj
    Dec 12, 2015 at 23:21

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